Optical filters have many applications in wavelength division multiplexing (WDM) systems, including but not limited to optical spectrum analysis, laser testing, wavelength demultiplexing, and amplified spontaneous emission filtering. Traditionally, optical filters have been based on fibre Bragg gratings (FBGs), thin film dielectric interference filters, Fabry-Perot filters, phased array waveguides, and diffraction gratings. The ideal optical filter is usually associated with flat channel passbands, low insertion loss, high isolation, low cost, high reliability and field ruggedness, high thermal stability, and good filter roll-off characteristics. Furthermore, in many instances it is preferable that the filter is tunable.
Referring to FIG. 1, there is shown a conventional tunable optical filter using a diffraction grating. The filter 2 includes an input optical fibre 4, an output optical fibre 6, a collimating/focussing lens 8, and a diffraction grating 10. The diffraction grating 10 is rotatable as shown in FIG. 1, in order to change the angle of incidence, and thus change the wavelength(s) that are filtered. Examples of this type of configuration are taught in U.S. Pat. Nos. 5,420,416, and 6,430,340 to Iida, incorporated herein by reference. Unfortunately, the diffraction grating used in these devices is typically polarization sensitive (i.e., the diffraction efficiency varies for light having different polarization states), and hence these types of filters typically exhibit significant polarization dependent loss (PDL).
U.S. Pat. Appl. 20002/0126385 proposes a configuration that reduces PDL utilizing a polarization beamsplitter and reflector for redirecting one of the beams of light transmitted from the polarization beamsplitter. Unfortunately, the polarization beamsplitter increases the cost and size of the device and further complicates optical alignment.
It is an object of the instant invention to provide a tunable optical filter with reduced PDL.
It is a further object of the instant invention to provide a tunable optical filter having increased resolution.